Multicoat coatings find diverse applications primarily in the coating of vehicles and the coating of films. In the case of automotive finishing, the system generally used is a multicoat coating system comprising an electrocoat, a surfacer, and a topcoat consisting of a basecoat and a clearcoat. Multicoat color and/or effect films which can be used, for example, to produce moldings generally comprise a support film, at least one basecoat and a clearcoat, which in terms of their composition typically correspond to the corresponding coating materials in automotive finishing.
Topcoat systems are typically applied by the wet-on-wet technique, in which the two coating materials are applied close to one another after flashing off (interim drying) of the basecoat material, but without interim crosslinking of the basecoat material, before then being cured together. Since, after the basecoat material has been flashed off (subjected to interim drying), it is not to be caused to bleed by the applied clearcoat material, both coating materials are generally based on solvents having strongly differing polarities. In this case, typically, aqueous basecoat materials and nonaqueous clearcoat materials are employed.
Presently, in order to reduce the emission of solvents, use is made of purely radiation-curable clearcoat materials and dual-cure clearcoat materials (clearcoat materials curable thermally and with actinic radiation), which are distinguished in particular by a low viscosity in conjunction with low solvent content in the uncured coating materials, and which possess properties that are good per se.
Thus, WO 2006/058680 A1 describes scratchproof coatings which are radiation-curable and may additionally be thermally curable and which are employed as topcoats, more particularly as or in automotive clearcoats and topcoats, and also as coatings on films. As synthesis components they comprise at least one compound having at least one isocyanate group, at least one compound containing at least one silicon atom and at least one isocyanate-reactive group, and at least one compound having at least one isocyanate-reactive group and at least one free-radically polymerizable group. Multicoat coatings in which the scratchproof coating can be used, however, are not described therein.
DE 10 2005 053 663 A1 describes aqueous, structurally viscous powder dispersions which are curable by free-radical polymerization and which have as their disperse phase particles having an average size of 80 to 750 nm, the particles containing at least one free-radically crosslinkable binder having a glass transition temperature of −70 to +50° C., an olefinically unsaturated double bond content of 2 to 10 eq/kg, and an acid group content of 0.05 to 15 eq/kg, in an amount, based on the particles, of 50% to 100% by weight. The powder dispersions can be employed in particular as clearcoat materials. Specific multicoat coatings in which the aqueous, structurally viscous powder dispersions curable by free-radical polymerization can be used, however, are not described.
WO 2006/067003 A1 describes multicomponent systems comprising (A) at least one component which contains isocyanate-reactive functional groups, is free from isocyanate groups, and contains functional groups with bonds which can be activated with actinic radiation, or is free from these functional groups. The systems further comprise (B) at least one component which contains isocyanate groups, is free from isocyanate-reactive functional groups, and optionally contains functional groups with bonds which can be activated with actinic radiation, with the proviso that at least one of the two components, (A) or (B), contains functional groups with bonds which can be activated with actinic radiation. Component (A) or at least one of the components (A) comprises at least one light stabilizer (L) selected from the group consisting of low molecular weight, oligomeric, and polymeric light stabilizers which contain at least one isocyanate-reactive functional group, and also (P) at least one photoinitiator which contains at least one isocyanate-reactive functional group. Dual-curable mixtures produced therefrom are suitable for producing films and moldings and also as coating materials, more particularly for clearcoats of multicoat paint systems. No further description is given of the basecoats which can be employed in such systems.
DE 10 2005 049 520 A1 describes a method of producing moldings from coated films, and also moldings produced therefrom. The films described in the publication are coated both with a pigmented coating material (P) and with a crosslinkable coating material (K) that comprises a free-radically crosslinkable component (KK) and which, after final crosslinking, produces a transparent coating (KE). This free-radically crosslinkable component (KK) comprises (i) one or more oligourethane and/or one or more polyurethane (meth)acrylates and (ii) on average more than one ethylenically unsaturated double bond per molecule, (iii) has a number-average molecular weight of 1000 to 10000 g/mol, (iv) a double bond content of 1.0 to 5.0 mol of double bonds per 1000 g of reactive component (KK), (v) on average >one branching point per molecule, (vi) 5%-50% by weight, based in each case on the weight of component (KK), of cyclic structural elements, and (vii) at least one aliphatic structural element having at least 6 C atoms in the chain, the free-radically crosslinkable component (KK) containing carbamate and/or biuret and/or allophanate and/or urea and/or amide groups. Besides the polyurethane (meth)acrylates, the free-radically crosslinkable component (KK) may also have constituents with other functional groups, such as hydroxyl, carboxyl, amino and/or thiol groups, for example. Use is additionally made, as a pigmented coating material (P), of a solventborne or aqueous coating material which typically comprises (I) one or more solvents and/or water, (II) one or more binders, preferably one or more polyurethane resins and/or one or more acrylate resins, and (III) if desired, at least one crosslinking agent, (IV) at least one pigment, and (V) if desired, one or more typical auxiliaries and additives.
WO 2006/117091 A1 describes a method of producing films coated first with a transparent coating and then with a pigmented coating, the films themselves, and their use in the production of moldings. This film can be peeled from the coatings. The crosslinkable coating composition (K) comprises a free-radically crosslinkable component (KK) which (i) contains one or more oligourethane and/or one or more polyurethane (meth)acrylates and which has (ii) a number-average molecular weight of 1000 to 50000 g/mol and (iii) a double bond content of 1.0 to 5.0 mol of double bonds per 1000 g of reactive component (KK), the free-radically crosslinkable component (KK) (iv) containing on average >1 branching point per molecule, (v) containing 5%-50% by weight, based in each case on the weight of component (KK), of cyclic structural elements, and (vi) having at least one aliphatic structural element with at least 6 C atoms in the chain, and (vii) containing carbamate and/or biuret and/or allophanate and/or urea and/or amide groups. These polyurethane (meth)acrylates may also contain other functional groups, such as hydroxyl, carboxyl, amino and/or thiol groups, for example. Suitable binders of the pigmented coating are the polyurethane resins and acrylate resins that are used typically in basecoat materials in the sector of the automobile industry. Additionally it is possible for the pigmented coating materials further to comprise at least one amino resin as crosslinking agent.
However, diverse requirements are imposed on the appearance of multilayer coatings, and are inadequately met by the systems known to date. In many cases, for instance, the adhesion of the clearcoat to the basecoat does not persistently meet the requirements imposed.